1. Field of the Invention
This invention is directed to a method for treating wastewater, such as blast furnace blowdown water, for removal of zinc and cyanide.
2. Description of Prior Art
In steel plant blast furnace technology, water is utilized to cool and cleanse gases coming out of the furnace. Such water is generally recycled, but a certain portion of this water is removed from the system for discharge into the environment. Such discharge water contains soluble zinc and cyanide which are objectionable. Such discharge water containing soluble zinc and free cyanide is referred to herein as blast furnace blowdown.
Environmental standards governing zinc and cyanide contents are stated in terms of total amounts of each element permitted in the discharged wastewater. Presently, the Environmental Protection Agency mandated maximum concentration for total zinc is 0.45 ppm for blast furnace blowdown. As one skilled in the art knows, a total amount of zinc is composed of that portion of zinc still in solution as a soluble compound (herein referred to as soluble zinc) plus that portion of zinc which is in solid precipitated form but still in suspension (referred to herein as suspended zinc). There are conventional means for effectively removing most but not all suspended zinc, such as high-rate filtration. Therefore, it becomes apparent that for a given inlet soluble zinc concentration, in order to reach the mandated total zinc standard, one must first reduce soluble zinc far enough below the total zinc standard so that any unremoved suspended zinc will not raise the total zinc to above the limit. Of course, the further below the total zinc standard a removal process lowers soluble zinc, the greater amount of suspended zinc permitted before the total zinc standard is reached. Therefore, there is a need for a process which removes soluble zinc at least to a level below total zinc standards (0.45 ppm) sufficient to permit the soluble zinc plus suspended zinc to be less than the total zinc standard.
A similar set of circumstances applies to cyanide. Presently, the Environmental Protection Agency mandated maximum concentration for total cyanide is 3 ppm for blast furnace blowdown. Cyanide which is soluble and available to react with zinc is referred to herein as free cyanide. Total soluble cyanide includes free cyanide (as defined herein) plus a soluble ferrocyanide and/or a soluble ferricyanide which, unlike free cyanide, are not available to react with zinc. Free cyanide will react with soluble zinc to form a zinc-cyanide complex, which is an environmentally harmful complex that is not removed from solution by traditional pH adjustment of the water.
One method for removing soluble zinc from acid steel plant wastewater consists of treating wastewater with a mixture of alkali-water slurry and recycled sludge formed later in the method. This mix neutralizes the acidity of the wastewater and precipitates a substantial portion of the zinc value as solids which settle out as a sludge, a portion of which is recycled as described above. Such process is described in commonly assigned U.S. patent application Ser. No. 291,439 filed Aug. 10, 1981, by Stewart T. Herman et al. The process described therein is referred to as a high density sludge process. Unfortunately, this process is not effective for treating zinc-cyanide complexes.
A method for removing cyanide which has been complexed with zinc in a wastewater is disclosed in U.S. Pat. No. 4,312,760 issued Jan. 26, 1982 to Roy G. Neville. The method disclosed therein requires the addition of ferrous sulfate in the presence of bisulfite ions. Neville discloses using a ferrous ion concentration of 200 to 300 ppm. Neville also requires the presence of bisulfite ions and the presence of an oil layer on top of the water to be treated to avoid oxidation of ferrous ion to ferric ion. Ferric ion will not react with complexed zinc-cyanide. It is, therefore, desirable to develop a process which does not require the expense and added step of bisulfite addition and an oil covering.